1. Field of the Invention
The present invention relates to a technique to prevent or reduce the adhesion of fine particles to the surface of a semiconductor substrate such as a semiconductor wafer during a process for manufacturing semiconductor devices.
2. Description of the Related Art
The level of integration in integrated circuits formed on the surface of a substrate such as a semiconductor wafer has been recently increasing. The line width of patterns has correspondingly become finer. The minimum fabrication size is 0.5 .mu.m in the next generation of 16 Mbit DRAM. It is considered that very small particles deteriorate the quality of the products and lower the yield of products in the manufacturing process.
Conventional techniques used to wash the surface of a substrate include a method of immersing a wafer into a mixture of ammonia water and hydrogen peroxide water, which is heated to about 80.degree. C. and a method of applying ultrasonic waves in ultrapure water as is described in RCA Review, 31 (1970) p. 187 to 206. Particularly, the latter ultrasonic washing method has been put to practical use by applying various modifications such as the use of 850 KHz ultrasonic waves in lieu of the more conventional 50 KHz ultrasonic waves as is described in J. Elec. Materials, 8 (1979) p. 855 to 864, changing the distance between an ultrasonic emitting surface and the surface top of the liquid as is described in Japanese Unexamined Patent Publication No. Sho 60-187380 and frequency-modulating a basic frequency as is described in Japanese Unexamined Patent Publication No. Sho 61-101283.
The minimum fabrication size in 16 Mbit DRAM is 0.5 .mu.m due an advancement in the level of integration in integration circuits. The particles of foreign matter which are to be washed away have correspondingly become smaller. It is considered that even particles of foreign matter as small as 0.05 .mu.m should be removed.
As the foreign matter particles to be considered become smaller, there are more types of foreign matter in the air.
The number of different types of foreign matter of 0.05 .mu.m size are twice as many as those of 0.13 .mu.m which should be currently removed for manufacturing 4 Mbit DRAM. It is considered that as the foreign matter becomes smaller, they are more likely to adhere to substrates. A technique for washing away very small particles of foreign matter has been increasingly in demand.
However, the physical energy applied upon one particle of foreign matter becomes smaller so that it is more difficult to remove the foreign matter from the substrate in accordance with conventional removal methods, for example, by applying a physical energy by ultrasonic waves. Damage to the substrate has become serious in association with an improvement in the level of integration of semiconductor devices. Accordingly, it is considered that it is difficult for conventional methods to remove very small particles of foreign matter.
The present invention was made under such circumstances.
It is an abject of the present invention to provide a technique to prevent or reduce the adhesion of fine particles to a substrate.